This invention relates to improvements in the design and operation of expanded or fluidised beds in which a fluid, particularly a liquid, is used to transmit the energy for bed expansion. An expanded or fluidised bed is one in which the particles are suspended in a fluid flow but do not substantially move with the bulk flow of that fluid. The classical chemical engineering definition of an expanded bed is one that is increased in volume up to 50 or 100% over that of the bed when static, i.e. with no fluid flow; whilst a fluidised bed is defined as having a volume more than 50 or 100% greater than that of the static bed with no fluid flow. In particular, it relates to biological processes such as water and wastewater treatment, fermentation, and bio-catalysis. For such processes, areas in need of improvement include distribution of the liquid flow; energy costs for pumping and aeration; control of biomass overgrowth; and biomass support materials.
Recent publications in the scientific literature have highlighted aspects of fluidised bed design and operation, which are in need of improvement. For example, P. M. Sutton and P. N. Mishra (“Activated carbon based biological fluidised beds for contaminated water and wastewater treatment: a state of the art review”, Water Science and Technology Vol. 29 10-11: 309-317, 1994) point out that “The mechanical components and sub-systems critical to the design of BFB (biological fluidised bed) commercial system embodiments are the following” and went on to cite the distributor, oxygen transfer, and control of biofilm growth. Their paper was based on a review of “Over 80 commercial, media-based BFB reactors (that) have been installed in North America and Europe.”
In a more recent review, C. Nicolella, M. C. M. van Loosdrecht and J. J. Heijnen (“Wastewater treatment with particulate biofilm reactors”, Journal of Biotechnology 80: 1-33, 2000) identified four key disadvantages of fluidised bed operation:    1. Biofilm formation on carriers, which poses problems leading to long start-up times.    2. Difficulty in control of biofilm thickness.    3. Overgrowth of biofilm leading to elutriation of particles.    4. High cost of liquid distributors for fluidised systems for large-scale reactors and associated problems with respect to clogging and uniform fluidisation.”
Since the introduction of liquid fluidised bed technology, a number of patents have been granted for devices to ensure the uniform distribution of liquid flow at the base of the bed. These include downward flow through expansion nozzles (e.g. U.S. Pat. Nos. 4,202,774, 4,464,262, 4,618,418, 5,584,996); nozzles with a perforated grid or plate above (U.S. Pat. Nos. 4,702,891, 4,933,149); perforated distributor plates, similar to those used in conventional gas-fluidised systems (U.S. Pat. No. 4,322,296) or with a static bed of coarse and fine grades of sand above (U.S. Pat. No. 5,965,016); or simply a static bed of granular material (U.S. Pat. No. 5,895,576), sand (GB780406) or both (GB2149683).
If the fluid flow at the base of the bed is turbulent, this results in increased impacts between fluidised particles producing abrasion, or in the case of particles carrying a reactant layer, premature stripping of the reactant layer from the fluidised carrier particles.
GB780406 discloses a particulate distributor comprising a static bed of sand lying on a perforated screen with flow rates of the order of 1 gallon per square foot per minute or about 0.08 cm per cm2 per sec. This low rate of flow through the distributor is insufficient to cause movement of its particles, and the teaching here is that the granular material is being used in effect as a “3-dimensional” perforated plate. In an attempt to improve the fluid flow characteristic in a fluidised bed, Bernard Suchowski, Joseph E. Gargas, Robert H. Hyde and Joseph Pluchino (U.S. Pat. No. 5,965,016); proposed the use of larger and heavier particles of sand collected just above a perforated distributor plate, where they help distribute the flow more evenly.
In spite of this, the presence of the perforated plate itself poses physical constraints to fluid flow.
We have found that by removing the plate completely and causing or allowing the particles of the distributor layer to move, but not themselves to be fluidised, a significant improvement in fluid flow properties in the lower part of the bed results.